1. Field of the Invention
The invention relates to an overlay mark and a method of forming the same.
2. Description of Related Art
To cope with the needs that electronic products are light, slim, short, and compact, sizes of electronic devices are reduced gradually. However, since the development speed of exposure apparatus is much slower than the speed at which the sizes are reduced according to market requirements, currently, a double patterning process is proposed. The double patterning process uses an existing apparatus to expose a photoresist pattern in a larger size and then forms a spacer with a smaller width on a sidewall of the photoresist pattern. After that, photoresist layers are removed. Since the width of the spacer may be adjusted to a required size according to actual needs through an etching process, the spacer left may serve as an etching mask to form a pattern in a required size.
Currently, an overlay mark is composed of four bar-shaped photoresist layers. Since the double patterning process removes the photoresist layers, only a circular spacer surrounding the four bar-shaped photoresist layers is left as an overlay mark. The circular spacer occupies a small area and is unable to provide sufficient contrast. Therefore, a signal of the circular spacer is very weak, and an alignment is not easy.